The invention relates generally to control systems for controlling the cyclical motion of fluidically driven members. More specifically, the invention relates to a control system for controlling straight-line shear for cutting gobs from a stream of molten glass in a glassware forming machine.
The use of shears for cutting gobs from a stream of molten glass falling from a feeder of a glassware forming machine is well-known in the prior art. Such shears are generally of either the arcuate or straight-line type, each type having a cyclically oscillating single pair of opposed blades associated with a single stream of molten glass. An example of a prior art double gob shear assembly of the straight-line type is shown in U.S. Pat. No. 4,174,647, dated Nov. 20, 1979, and assigned to the assignee of the present invention. Prior art shears are generally activated by pneumatic (or fluidic) means interconnected in a complex arrangement of various valves and cylinders.
The present invention relates to an improvement over the prior art control system of commonly assigned U.S. Pat. No. 4,467,431, the disclosure of which is incorporated by reference herein. Such control system for pneumatically driven straight-line shears (such as shears of the type disclosed in the '647 patent cited above), such control system incorporating a series of sensors which produce signals representing predetermined shear positions in each shear cycle. As illustrated in FIGS. 5 and 6 at the '431 patent and the text discussing these Figures, such prior art system incorporates four sensors--a return delay sensor, a zero crossover sensor, an overlap sensor, and a cushion sensor, each of which produces an output signal at a predetermined point of the travel of the shears, for controlling various aspects of the shears, operation in accordance with the method disclosed therein. The shears are pneumatically driven in the forward (cutting) and reverse direction by selectively energizing and deenergizing (or reversing) the cylinder assembly of FIG. 3. The timing of this energization and denergization, and hence the motion profile of the shears, is determined by the timing of the return sensor and overlap sensor signals, as well as by a return delay time which is added to the return sensor signal to determine the off time of the shears' pneumatic drive.
That aspect with which the present invention is particularly concerned, i.e. the synchronization of the shears motion with other mechanisms such as a Feeder assembly, is discussed at column 7, line 31--column 10, line 28, with reference to FIGS. 8 and 9 of the '431 patent. The Start Delay Timer circuit 16 of '431 FIG. 8 is included herein as FIG. 3 for convenient reference. The set-up circuit 18 of FIG. 9 modifies the operation of the start-delay timer 16 of FIG. 8 during initialization, in order to approximate in the latter circuit the conditions of actual operation. Set-up circuit 18 causes the up-down counter 312 (FIG. 8) to count up during the period between first and second "sync pulses", and down for a prescribed time period ("shear response time"). Thus, with particular reference to the prior art shears displacement plot of FIG. 1 herein, the count in up-down counter 312 represents the "delayed start time" 230 between the "on" time 204 of each cycle and the sync pulse of the preceding cycle in the upper of the two start/sync. timing diagrams. This sync pulse is aligned with the zero crossover point 12 and the Shear Response Time 220 was defined as time between energizing the shear mechanism (on time 204) and the zero crossover point 220.
The start delay timer 16 of the system of U.S. Pat. No. 4,467,431 is designed to maintain the leading edge of the sync pulse (beginning of "start delay time38 ) with the leading edge 212 of the zero crossover signal. Up/down counter 312 (defining the "start delay time38 ) will be either incremented or decremented by a single clock pulse each cycle depending on whether the sync pulse follows or precedes the zero crossover signal.
As an improvement to the assignee's commercial system based upon the invention of the '431 patent, the circuits of '431 FIGS. 8 and 9 were chronize the start time to the leading edge of the return sensor signal (point 206 in FIG. 1) rather than that of the zero crossover signal. This is illustrated by the "modified start/sync." timing diagram of FIG. 1. In this modification, the shear response time 220' was measured from the on time 204 and the leading edge of the Return Sensor Signal, and the Start Delay Time 230', occupied the balance of the cycle. This arrangement improved the reliability of synchronization, in that point 206 is at a more uniformly increasing portion of the displacement profile. However, this system suffered the same shortcoming as encountered in that of the '431 patent--i.e. that in the event of a significant shift of the speed of the mechanism from which the start pulse was obtained, such as a glass gob feeder assembly, the shears sometimes required hundreds of cycles to achieve the appropriate new Start Delay time.
Accordingly, it is the principal object of the invention to provide an improved system for synchronizing the cyclical actuation of a fluidically driven shear mechanism to a related mechanism such as a glass gob feeder. As a related object, such improved synchronizing system should be adaptable to other fluidically driven mechanisms of a glassware forming machine.